@MasterPie: A simple solution would be a "button" similar to the Win7 "show desktop" button. Relatively small, unobtrusive, but it gives the necessary visual cues and gives you a hit target large enough for mouse usage in these situations. I expect (read "hope for") these small sort of UI fixes in Blue.
It's called the Start button. Seriously though just adding some chrome like a start button that invoked the start screen would go a long way IMO. On the Windows Store App front adding a "close" x in the upper right corner when the mouse is detected and/or moved would be a nice crutch for those coming over from the desktop world still stuck on mouse & keyboard (and insistent on closing their apps).